Process to preheat and carburate directly reduced iron (DRI) to be fed to an electric arc furnace (EAF)

ABSTRACT

The present invention relates to a high energy-saving process for preheating and carburation of directly reduced iron (DRI) to be conveyed to a melting process, by using melting furnace off-gas, which is characterized by a high temperature and a high CO-content.

[0001] The present invention relates to a high energy-saving process for preheating and carburation of directly reduced iron (DRI) to be conveyed to a melting process, by using melting furnace off-gas, which is characterized by a high temperature and a high CO-content.

FIELD OF THE INVENTION

[0002] According to the state of the art, steel can be produced from iron ores/iron minerals by direct reduction of the solid iron ore to produce sponge iron (“directly reduced iron”, DRI). This DRI is then fed to a melting furnace or a smelting furnace, for example, an electric arc furnace (EAF), or the like. Due to the need of reducing steel production costs, research directed towards these integrated processes that relate to the production of liquid steel from Fe mineral, has increased.

[0003] The only integrated process which has found an industrial application is the one quoted in U.S. Pat. Nos. 5,447,550 and 5,296,015, which essentially concerns the production of DRI in a shaft through hot discharging, pneumatic transportation and feeding to an EAF. In order to make the continuous DRI production process compatible with the discontinuous EAF process, a part of the DRI product is cooled and stored in a parallel circuit. Obviously, the presence of a reduction shaft in proximity of the melting process zone is required.

[0004] Principally, heat can be transferred from a gas to an object through two different ways. First, heat can be transferred via the sensitive power/heat of a gas, which corresponds to its temperature, flow rate and composition. Secondly, the latent heat/power of a gas can be used, for example in an EAF off-gas, which has a high CO-content, the heat generated by the exothermic oxidation reaction of CO: CO+½O₂=>CO₂.

[0005] The processes disclosed in the U.S. Pat. Nos. 5,447,550 and 5,296,015 enable to recover the sensitive heat of the charged DRI, but present some disadvantages that make it non-competitive from an economical point of view, especially as regards high productivity. In fact, besides the difficulties associated with the utilisation of pneumatic transport, i.e. high percentage of fines in the product, the necessity of a treatment gas circuit to avoid the DRI reoxidation, the main problem is the high power (sensitive power+latent power) dissipated by the EAF off-gases.

[0006] Another method that allows some energy saving in the melting process consists in preheating the EAF feeding (i.e. the DRI) by means of the sensitive heat of the gases coming out of the melting process. There are several patents related to this method, for example U.S. Pat. No. 4,736,383, WO 98/43032 or U.S. Pat. No. 4,002,465, which is mainly used for scrap pre-heating and which is suitable for different plants. However, all these processes are associated with high emissions of CO₂.

[0007] A very important parameter for the DRI composition in the production of liquid steel concerns the C content (carburation): the optimal theoretic C-content value is the one which facilitates or enables the reduction of the remaining FeO (wustite) during the melting process in the EAF, according to the following reaction:

FeO+C=>Fe+CO

[0008] Thus, it would be desirable to adjust the C-content of the DRI before transferring the DRI to a melting furnace. Carburation is generally carried out in the mineral reduction reactor disclosed in U.S. Pat No. 4,897,113, No. 4,734,128, No. 4,752,329. Although carburation can be achieved in this manner, it is associated with a rather high consumption of CH₄. In order to achieve an increase the C-content of 1%, 18-20 Nm³ CH₄ per ton DRI have to be consumed.

[0009] Therefore, it is an object of the present invention to provide a process for preheating and carburation of directly reduced iron (DRI) to be conveyed to a melting furnace which does not have the above-mentioned disadvantages and which considerably reduces the energy consumption in steel production.

[0010] These problems are solved by a process according to claim 1 for preheating and carburation of directly reduced iron (DRI) to be fed to a melting furnace, preferably an electric arc furnace (EAF). The process according to the present invention has the advantage that both the latent heat and the sensitive heat of the melting furnace off-gas are used to preheat the DRI. Further, due to the high CO-content of the melting furnace off-gas, natural gas for carburation of said DRI is saved and the addition of carbon in the melting furnace or the EAF can be avoided or at least considerably reduced.

[0011] In a preferred embodiment of the present invention at least the temperature of the melting furnace off-gas is measured before adjusting the temperature and the composition of the melting furnace off-gas.

[0012] In another preferred embodiment at least the composition of the melting furnace off-gas is measured before adjusting the temperature and the composition of the melting furnace off-gas.

[0013] According to the present invention it is preferred that the melting furnace is an electrical furnace (EAF).

[0014] The values of temperature, pressure and composition of the off-gases are measured and the temperature and CO/CO₂-ratio of said gas are adjusted in order to obtain a composition that may be utilized for preheating and carburization of DRI.

[0015] In the process according to the present invention, it is preferred that the temperature of the melting furnace off-gas is adjusted to ≦800° C., preferably 450-800° C., and/or its composition to have a CO/CO₂-ratio of at least 1, preferably 1-3. The upper limit of 800° C. is determined by the fact that usually the CO/CO₂-ratio in the off-gas coming from the furnace is not over 5,5. This value corresponds to a temperature of 800° C. for the carburation zone (see FIG. 2A). The lower limits of 450° C. for T and 1,0 for the CO/CO₂-ratio are determined by the fact that below these values no remarkable deposition of C is verified due to the kinetic of carburation.

[0016] The adjustment of temperature and composition of the melting furnace off-gas is preferably performed by burning a mixture of air and/or O₂, recycle gas obtained from gas which has been contacted with the DRI, and optionally CH₄, with the melting furnace off-gas.

[0017] The recycle gas is preferably obtained after removing fines, cooling the gas, pressurizing and intermediate storage of the gas.

[0018] The DRI is preferably preheated to a temperature of 300-700° C. using the adjusted melting furnace off-gas. The use of melting furnace off-gas has the advantage that the melting furnace off-gas does not have to be transferred to a fume plant and wasted.

[0019] In a preferred embodiment the DRI is contacted with said adjusted gas by a vertically split flow.

[0020] According to the present invention the DRI is preferably carburated to a C-content of 0-3 wt-%. Thus, the addition of carbon in the melting furnace or EAF can be avoided or at least considerably reduced.

[0021] It is further preferred that the adjusted gas contains 0-30% H₂ which is produced by the reaction of CH₄ with O₂. The presence of H₂ has the advantage that the reducing capability of the atmosphere used to preheat the DRI is increased, which prevents Fe in the DRI from being (re)oxidized to FeO and/or helps to reduce residual FeO in the DRI to Fe.

[0022] The present invention also comprises a plant for performing the process according to claim 1.

[0023] It is preferred that the plant for performing the process of the present invention comprises a means for measuring at least the temperature of the melting furnace off-gas and/or the composition of the melting furnace off-gas. This means is preferably a combustion chamber.

[0024] In said plant, the adjustment of the melting furnace off-gas is preferably performed in a combustion chamber, comprising a controlled burner, controlled inlets for melting furnace off-gas, CH₄, air and/or O₂ and cooling recycled gas and a controlled outlet for transferring said adjusted gas to the preheater.

[0025] In a preferred embodiment, the plant further comprises a device for separating fines from the preheater off-gas, a cooling device for cooling the preheater off-gas, a pressurizing device and an intermediate storage device, from which the recycled preheater off-gas is transferable to the combustion chamber and/or a fume plant. Thus, the preheater off-gas can be advantageously recycled and used to cool the melting furnace off-gas.

[0026] The preheater comprises preferably at least one discharging device at the bottom and at least one gas inlet for the gas to be contacted with the DRI.

[0027] It is further preferred that the preheater has essentially a cylindrical form.

[0028] In a preferred embodiment, the preheater comprises at least two conical discharging devices located at the bottom of said cylindrical container and three gas inlets for the gas to be contacted with the DRI, wherein one gas inlet is located centrally at the bottom of the preheater and two gas inlets are located at the side of said conical discharging devices. This construction has the advantage that the DRI can be efficiently contacted with the adjusted melting furnace off-gas.

FIGURES

[0029]FIG. 1 shows the thermodynamic flow of the Boudouard reaction as a function of the temperature for a feeding gas characterized by a CO/CO₂-ratio of 2 at a temperature of 750° C. or higher.

[0030]FIG. 2A shows the thermodynamic conditions for reduction and carburation in a Fe—C—O thermodynamic diagram (CO+CO₂=100%).

[0031]FIG. 2B shows the thermodynamic conditions for reduction and carburation in the presence of 20% (CO+CO₂).

[0032]FIG. 3 shows a plant for preheating and carburation of DRI to be fed to an EAF.

[0033]FIG. 4 is an enlarged view of the preheater of FIG. 3.

[0034]FIG. 5 shows a side view of the preheater and sectional views indicating the positions of gas inlets.

DETAILED DESCRIPTION

[0035] The present invention describes a procedure which permits to preheat and carburate DRI using the off-gas of the melting process, usually performed in an EAF. According to this process, the energy consumption in the production of liquid steel can be considerably reduced in different ways:

[0036] utilisation of latent heat and sensitive heat of the melting furnace off-gas;

[0037] reduction of natural gas (CH₄) consumption for the carburation;

[0038] addition of carbon in the melting furnace or the EAF can be avoided or at least reduced.

[0039] Carburation can be carried out in a preheating reactor thanks to the exothermic Boudouard reaction (2CO

C+CO₂) which is catalysed by Fe and whose thermodynamic flow is shown in FIG. 1 for a gas having a CO/CO₂-ratio of 2 at a temperature of 750° C. or higher. Although the C deposit reaction is favoured by low temperatures (exothermic reaction), it does not show a remarkable reaction rate for T<400-500° C.

[0040] It is possible to use different types of melting furnaces, for example smelting furnaces. However, the use of an EAF is preferred in the present invention.

[0041] The composition of the gas fed to the preheating reactor and its temperature should be defined in order to avoid the Fe reoxidation reaction and, if possible, to favour the reduction of remaining FeO present in the DRI. In FIG. 2A, a thermodynamic diagram is shown, which gives the min. value of the CO/CO₂-ratio to be present in the gas to heat the DRI without incurring in its reoxidation. The limit conditions are also summarized in Table 1. The components of the off-gases are in this case only CO and CO₂. In the real case, off-gases are composed of CO, CO₂ and other components, i.e. N₂, H₂, H₂O. In this case, the C+CO₂

2CO equilibrium curve moves towards left (see FIG. 2B for a (CO+CO₂) content of the gas phase of 20%), fixing new limit conditions. TABLE 1 Limit conditions for obtaining preheat-reduction and/or preheat- carburation CO/CO₂-ratio T/° C. FeO reduction/Formation of CO 1.6 730 Lower limit FeO reduction/Carburation 1.6 730 Lower limit Fe oxidation/Carburation 1.6 730 Higher limit Fe oxidation/Formation of CO

[0042] Although the melting furnace off-gas composition will usually be adjusted using recycle gas, CH₄ and O₂ and/or air, it is also possible according to the present invention to use the melting furnace off-gas without a subsequent adjustment of its composition.

[0043] The off-gas of the melting process is preferably conveyed into a combustion chamber where, after mixing with recycle gas and, if necessary, with CH₄ and O₂ and/or air (in order to increase the CO/CO₂ ratio), a reducing gas with desired temperature and composition is generated. Cold fed DRI can be preheated, depending on the gas used, up to a temperature of 300-700° C. In this case, the preheating is to a large extent achieved by the heat generated by the exothermic carburation reaction.

[0044] The upper limit of the DRI preheating temperature depends on the characteristics of the processed material, but may be limited by the following considerations:

[0045] a too high temperature could lead to DRI pellet sticking;

[0046] a faster Fe re-oxidation kinetic occurs at high temperatures, even at high values of the CO/CO₂-ratio;

[0047] C, initially deposited on the DRI according to the Boudouard reaction:

[0048] 2CO

C+CO₂ may be (re)gasified.

[0049] Generally, the following conditions are applicable to achieve a combined preheating and carburation:

T_(inlet gas)≦800° C. CO/CO₂≧1 (compare FIG. 2A)

[0050] Preferred operative ranges to obtain preheating and carburation are:

T_(inlet gas)=450° C.-800° C. with CO/CO₂=1-3.

[0051] The DRI is preheated to a temperature of 300 to 700° C.

[0052] It is possible in the process according to the present invention to obtain a C-content in the DRI up to CDRI=0-3%

[0053] Short Description of the Plant

[0054] A suitable plant according to the present invention should focus on the possibility of carburation and preheating DRI using melting furnace off-gases, preferably EAF off-gases, which otherwise would directly flow to the fume plant.

[0055] In such a plant, the sensitive heat of fumes with a rich carbon monoxide content is used, in addition with the heat produced by their combustion with air or an air/O₂-Mixture in a chamber capable of burning such fumes in a controlled manner. If necessary, CH₄ and O₂ can be injected in the combustion chamber in order to increase the CO/CO₂-ratio and to obtain a reducing atmosphere through the reaction of partial CH₄ oxidation to yield CO and H₂.

[0056] The combustion products exchange their sensitive and latent heat with the DRI in a preheater which is charged and operated seperately from the EAF. Dust is removed from fumes leaving the preheater, which are then cooled, passed through a booster and afterwards conveyed to the above-mentioned combustion chamber.

[0057] The proportion of cold fumes, which has not been used in the closed process cycle, is conveyed to an exhaust gas treatment plant.

[0058] In a variant of the plant, the gases coming from the preheater are directly conveyed to the fume plant. In this case the temperature of the off-gases flowing to the preheater may be controlled, for instance, using a heat exchanger. In any case the plant is equipped with instruments for measuring the temperature and the composition of the melting furnace off-gas.

[0059] The range of flow rates of the adjusted off-gas to be fed to the preheater is very wide. The lower value is the theoretic value required for the carburation and preheating of the DRI. The maximum value depends on the pressure drop in the preheater and on the stability of the material inside the preheater. Flow rates range from about 300 to about 2,500 Nm³/t.

[0060] In the following example a plant and a process according to the present invention are described. However, the example is not intended to limit the scope of the present invention.

EXAMPLE

[0061] A plant in accordance with the present invention is shown schematically in FIG. 3. The plant includes an EAF (1), capable of melting DRI for the production of liquid steel. The secondary product of the melting operation (EAF off-gas) is the off-gas of the decarburation phase. Since it is mainly made of CO, it can bum and produce the heat necessary to preheat the DRI.

[0062] This off-gas leaving the EAF with a temperature of about 1200° C. to about 1600° C. enters the combustion chamber (2), where it is oxidised with air (air/O₂) in an atmosphere thermally controlled by the recycle gas according to a ratio, which permits to obtain fumes at a temperature of <800° C. and a CO/CO₂-ratio of 1-3. If necessary, CH₄ and ° 2 can be injected in the combustion chamber in order to increase the CO/CO₂-ratio and to obtain a reducing atmosphere through the reaction of partial CH₄ oxidation to yield CO and H₂. The H₂-content can vary between 0 to about 30%.

[0063] Since the combustion products in the following heating phase enter in straight contact with the DRI, gases must be devoid of oxidant characteristics, and in order to achieve this, the CO/CO₂-ratio has to be regulated by acting on the combustion air flow.

[0064] The fumes that leave the combustion chamber enter the preheater (3) through some distributors at the bottom of the cylindrical area.

[0065] The preheater (3) can be seen as a shaft for inside refractory preheating and is equipped with a tapered discharging device at the bottom for the descent of the solid material once the heating has concluded. While ascending, the adjusted melting furnace off-gas transfers its heat to the DRI which is loaded in solid form at ambient temperature prior to the preheating by means of a charging system, and leaves the preheater through a collector located at the upper side of the preheater (3). Afterwards, the gases pass to the operations of removement of the fines and a dust separator (4) (cyclone and scrubber) and are subsequently cooled by means of a heat exchanger (5). The gas temperature reached at the end of these treatments allows using a fan (6) capable of adjusting the pressure back to the values approximating the ones present in the combustion chamber. The fan performance does not exceed 600-800 mm H₂O.

[0066] The gases thus compressed are pushed into a storage unit (7) with the aim of filling the flow gas power, if any, during start-ups and preheating stoppages.

[0067] The off-gas of the storage unit (7) is split by a three-line valve, regulated by controlling the temperature of the products of the fume partial combustion of chamber (2), which divides the off-gas into two independent flows: the first one is directed to the combustion chamber, the second one is destined to the exhaust gas treatment plant.

[0068] The preheater, which is shown in FIG. 4, is not very high and has a considerable horizontal extension. Thus, a specific gas distribution is required to homogeneously contact all the charged material mass. The heating gas is injected through two distinct ways: the first one consisting in an injection between the two discharge cones at the bottom of the cylindrical area (8), the second one through two distribution rings placed at the top of the two discharge cones (9). In FIG. 5, sectional views of the preheater are shown, indicating the positions of the gas inlets. 

1. A process for preheating and carburation of directly reduced iron (DRI) to be fed to a melting furnace, including the steps of: adjusting the temperature and the composition of melting furnace off-gas outside the melting furnace to a value, suitable to preheat and carburate the DRI, contacting the DRI with said adjusted gas to preheat and carburate the DRI.
 2. A process according to claim 1, characterized in that at least the temperature of the melting furnace off-gas is measured before adjusting the temperature and the composition of the melting furnace off-gas.
 3. A process according to claim 1, characterized in that at least the composition of the melting furnace off-gas is measured before adjusting the temperature and the composition of the melting furnace off-gas.
 4. A process according to one or more of the preceding claims, characterized in that said melting furnace is an electric arc furnace (EAF).
 5. A process according to one or more of the preceding claims, characterized in that said melting furnace off-gas is adjusted to a temperature of ≦800° C. and a CO/CO₂-ratio of at least
 1. 6. A process according to one or more of the preceding claims, characterized in that the temperature of said melting furnace off-gas is adjusted to 450-800° C.
 7. A process according to one or more of the preceding claims, characterized in that the composition of said melting furnace off-gas is adjusted to have a CO/CO₂-ratio of 1-3.
 8. A process according to one or more of the preceding claims, characterized in that the temperature of said melting furnace off-gas is adjusted to 450-800° C. and its composition to have a CO/CO₂-ratio of 1-3.
 9. A process according to one or more of the preceding claims, characterized in that the adjustment of temperature and composition of said melting furnace off-gas is performed by burning a mixture of air and/or O₂, recycle gas and optionally CH₄, with the melting furnace off-gas.
 10. A process according to claim 9, characterized in that said recycle gas is obtained from gas, which has been contacted with the DRI in the preheating and carburation step (preheating off-gas).
 11. A process according to claim 9 or 10, characterized in that said recycle gas is obtained after removing fines, cooling the gas, pressurizing and intermediate storage of the gas.
 12. A process according to one or more of the preceding claims, characterized in that said DRI is contacted with said adjusted gas by a vertically split flow.
 13. A process according to one or more of the preceding claims, characterized in that said DRI is preheated to a temperature of 300-700° C.
 14. A process according to one or more of the preceding claims, characterized in that said DRI is carburated to a C-content of 0-3 wt.-%.
 15. A process according to one or more of the preceding claims, characterized in that said adjusted gas contains 0-30% H₂, which is produced by the reaction of CH₄ with O₂.
 16. A process according to one or more of the preceding claims, characterized in that the temperature of said melting furnace off-gas is about 1200° C.-1600° C. before adjustment thereof.
 17. A plant for performing the process according to claim 1, comprising: a melting furnace, a means for the adjustment of the melting furnace off-gas, and a preheater for contacting the DRI with said adjusted melting furnace off-gas.
 18. A plant according to claim 17, characterized in that said means for the adjustment of the melting furnace off-gas is a combustion chamber.
 19. A plant according to claim 17 or 18, further comprising means for measuring at least the temperature of the melting furnace off-gas.
 20. A plant according to claim 17, 18 or 19, further comprising means for measuring at least the composition of the melting furnace off-gas.
 21. A plant according to one or more of the claims 17 to 20, characterized in that said melting furnace is an electric arc furnace (EAF).
 22. A combustion chamber, comprising a controlled burner, controlled inlets for the off-gas, CH₄, air and/or O₂ and cooling recycled gas and a controlled outlet for transferring said adjusted gas to the preheater.
 23. A plant according to one or more of the claims 17 to 21, characterized that said plant further comprises a device for separating fines from the preheater off-gas, a cooling device for cooling the preheater off-gas, a pressurizing device and an intermediate storage device, from which the recycled preheater off-gas is transferable to the combustion chamber and/or a fume plant.
 24. A plant according to one or more of the claims 17 to 21 or 23, characterized in that said preheater comprises at least one discharging device at the bottom and at least one gas inlet for the gas to be contacted with the DRI.
 25. A plant according to one or more of the claims 17 to 21, 23 or 24, characterized in that said preheater has essentially a cylindrical form.
 26. A plant according to one or more of the claims 17 to 21, 23, 24 or 25, characterized in that said preheater comprises at least two conical discharging devices located at the bottom of said cylindrical container and three gas inlets for the gas to be contacted with the DRI, wherein one gas inlet is located centrally at the bottom of the preheater and two gas inlets are located at the side of said conical discharging devices.
 27. Use of the process according to claim 1 in the production of steel. 